* @ORM\JoinTable(name="user_group",
* joinColumns={@ORM\JoinColumn(name="user_id", referencedColumnName="id")},
* inverseJoinColumns={@ORM\JoinColumn(name="group_id", referencedColumnName="id")}
* @ORM\JoinTable(name="user_group",
* joinColumns={@ORM\JoinColumn(name="user_id", referencedColumnName="id")},
* inverseJoinColumns={@ORM\JoinColumn(name="group_id", referencedColumnName="id")}